Friday, April 07, 2006

Bio vs. The Plug

A pair of articles in this week's New York Times "Drive Times" e-newsletter frame an interesting energy dilemma. The first piece provides more detail on the "plug-in" hybrid cars being advocated by many commentators. The Times focuses on who makes them today--as modifications to existing hybrid models--and how some of the original equipment manufacturers view that. The other article, from, describes Saab's ethanol-powered all-wheel-drive hybrid prototype. Between them, they illustrate a central dilemma for alternative energy planners: upon what primary energy sources should future cars draw, other than petroleum?

The future energy and environmental impact of transportation energy will hinge on whether motive power is generated onboard or remotely, and from which fuel. The plug-in hybrid answers that in two ways, by relying on an internal combustion engine and by storing up power from the grid. That means its primary fuel is supplemented by some combination of coal, natural gas, nuclear, hydroelectric, and wind, depending on the local power grid's dispatch curve. Note that many of these fuels result in additional, offline emissions of both local pollutants and greenhouse gases, in addition to the emissions from the car's engine.

The "bio hybrid" represents a different philosophy, opting for all-onboard power, but utilizing a biofuel with minimal net greenhouse gas impact. Grain-based ethanol only returns 20% more energy than went into its production, and burning it in an internal combustion engine is less efficient than producing electricity from natural gas in a combined-cycle gas turbine. But despite this, it is not an inherently worse pathway than the plug-in hybrid. Depending on the mix of natural-gas-derived fertilizer and fossil-fuel based process heat that went into making it, it may still produce fewer of the emissions that are linked to climate change than the gasoline and grid-based electricity supply for the plug hybrid.

Even if you fill up the plug-in hybrid with ethanol or some other biofuel, you can't escape the environmental consequences of its external power source. Weighing those against a pure bio-hybrid requires a detailed well-to-wheels analysis, for each market in which the cars would be sold. That's orders of magnitude more complicated than the miles per gallon testing currently done for each car model and powertrain combination. If nothing else, this is a good illustration of just how murky and confusing our future energy choices may get. Although aiming at global problems, the answers could still vary considerably, based on local conditions.

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